Some known piezoelectric devices are made from ceramics such as PZT (solid solution of lead titanate (PbTiO.sub.3) and lead zirconate (PbZrO.sub.3)). Other known piezoelectric devices are made from high-molecular materials such as PVDF (polyvinylidene fluoride). These piezoelectric devices find extensive use as devices for generating sound from the audible range to the ultrasonic range, as electromechanical transducers such as actuators and motors, and as mechano-electrical transducers such as pressure sensors.
A conventional sound-generating device comprises a vibration source such as the above-noted piezoelectric device and a Helmholz resonance box. Vibration of the vibration source is resonated by the resonance box to generate large sound which is emanated from a hole of the resonance box.
Piezoelectric devices made from ceramic materials must be sintered at high temperatures of about 1000.degree.-1500.degree. C., and therefore it is difficult to obtain dimensional accuracy. Also, ceramic materials are very brittle and so they break easily. For piezoelectric devices made from high-molecular materials, high-molecular materials formed in a film-like shape are mechanically stretched. So that, it is difficult to obtain dimensional accuracy. Known piezoelectric devices must be subjected to poling process, in which a high DC electric field is applied at Curie temperatures or above, and then they are cooled below Curie temperatures to align the electric dipoles, in order to develop piezoelectric property. Thus, the manufacturing processes are troublesome.
As for the prior art sound-generating devices using such piezoelectric devices that employ resonance boxes, it is difficult to fabricate them in small sizes. Especially, it is difficult to make them thin. Since sound emanates from a hole formed in a resonance box, sound propagates in only certain directions. Thus, it has been impossible to emanate sound in every direction. Further, limitations are imposed on the degree of freedom given to the shape. This makes it difficult to produce loud sound.